Since the introduction of chromosomal banding techniques in the early 1970's, numerous tumors have been associated with specific chromosomal defects. Prezygotic chromosomal deletions have been causally related to two specific childhood tumors (retinoblastoma and Wilm's), while a large number of hematopoietic neoplasms have been associated with site-specific translocations or complete or partial monosomy. Less is known about the chromosome abnormalities of solid tumors because it has been technically difficult to obtain chromosome preparations from them, but there is evidence of specific changes in solid tumors as well. Thus, we are confronted with many striking examples of the relationship between chromosome structure. molecular regulation. gene expression and oncogenesis. To gain further insight into this relationship, the highest levels of sophistication in chromosomal, molecular and gene mapping techniques are required. In recent years, geneticists have employed the procedure of somatic cell hybridization for studies of gene control and gene mapping. Cells from two species are fused to form a single cell, and in the process chromosomes are lost. Clones developed from a hybrid retain a set of chromosomes characteristic of the clone, and one can correlate gene expression or presence of a specific DNA sequence with the presence or absence of that particular chromosome in a panel of hybrid clones. Numerous genes have been mapped using this method, and success relies heavily upon cytogenetic evaluation of the hybrid clones. Of all the gene mapping procedures, however, the one with the greatest potential is in situ hybridization. With this method, it is now possible to directly map any DNA sequence, even if it does not code for an identifiable gene product. Therefore, it is not a coincidence that the elements which contribute to this confluence of cytogenetics and molecular biology were put in place in the cytogenetics laboratory at The Children's Hospital of Philadelphia. The ability to dissect the chromosome at both the cytogenetic and molecular levels will rely upon services offered by the Cytogenetics Core.